1. Field of the Invention
The present invention relates to a rush current limitation and noise reduction circuit for a charge pump for limiting a rush current in a PFM (Pulse Frequency Modulation) control charge pump circuit and for reducing a noise.
2. Description of the Related Art
Up to now, in a PFM control charge pump circuit shown in FIG. 2, upon activation of a power supply or activation of the operation (ON-time) of the charge pump circuit, a large rush current is caused to flow into a pump capacitor or an output capacitor from an input power supply. In addition, in the case of the PFM control, even in the normal oscillation cycle, the change in input voltage increases a peak current during the operation so that the GND potential varies, which becomes the source of generation of a noise.
However, in the conventional PFM control charge pump circuit, a large rush current is caused to flow upon activation of a power supply. Here, a charge pump circuit 15xe2x80x2 includes driver circuit 5 for supplying the electric charges to a pump capacitor 4 and an output capacitor 3, respectively, driver circuit 9 for driving the driver circuit 57 respectively, and a PFM control circuit 6. In addition, since even during the normal operation, the change in input voltage causes a large current to flow, such a large current causes the voltage drop of an input power supply and becomes the source of generation of a noise. As a result, there is a possibility that other circuits connected to the same input power supply as that of the charge pump circuit undergoes malfunction.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to provide a rush current limitation and noise reduction circuit for a PFM control charge pump which is capable of suppressing generation of a rush current and a noise to prevent other circuits connected to the same input power supply as that of a PFM control charge pump circuit from undergoing malfunction.
In order to solve the above-mentioned problems, according to the present invention, there is provided a voltage detecting circuit for detecting an input voltage inside a PPM control charge pump circuit, in which potential differences between potentials appearing at gate terminals and potentials appearing at source terminals are reduced by, in response to a signal from the input voltage detecting circuit, suppressing gate voltages of switch transistors of a charge pump to suppress a rush current value to thereby reduce a current to prevent generation of a noise.
According to the present invention, a voltage detecting circuit for detecting an input voltage is provided inside a PFM control charge pump circuit, and also in response to a signal from the input voltage detecting circuit, a gate voltage of a switch transistor in the charge pump circuit is suppressed. As a result, even if the input voltage varies, a rush current is held to a minimum to suppress a current to thereby suppress generation of a noise. Consequently, there is also removed a possibility that other circuits connected to the same input power supply as that of the charge pump circuit undergo malfunction.